Sea slug masses migrate to Northern California coast

Updated 1:55 pm, Monday, February 2, 2015

Scientists have reported densities of up to dozens of inch-long nudibranchs or sea slugs, like this one, per square meter in tide pools from San Luis Obispo to Humboldt counties.

Scientists have reported densities of up to dozens of inch-long nudibranchs or sea slugs, like this one, per square meter in tide pools from San Luis Obispo to Humboldt counties.

Photo: Gary McDonald / Courtesy Of UC Santa Cruz

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Scientists have reported densities of up to dozens of nudibranchs, like this one, per square meter in tide pools from San Luis Obispo to Humboldt counties.

Scientists have reported densities of up to dozens of nudibranchs, like this one, per square meter in tide pools from San Luis Obispo to Humboldt counties.

Photo: Jeff Goddard / Courtesy Of UC Santa Cruz

Image 3 of 3

Scientists have reported densities of up to dozens of nudibranchs, or sea slugs, per square meter in tide pools from San Luis Obispo to Humboldt counties. This photo of multiple sea slugs was taken Jan. 19 at Hazard Canyon Reef near Montaña de Oro State Park. less

Scientists have reported densities of up to dozens of nudibranchs, or sea slugs, per square meter in tide pools from San Luis Obispo to Humboldt counties. This photo of multiple sea slugs was taken Jan. 19 at ... more

Photo: Jeff Goddard / Courtesy Of UC Santa Cruz

Sea slug masses migrate to Northern California coast

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A colorful flood of tiny southern sea slugs rarely seen in the waters off Northern California is puzzling scientists concerned about the warming ocean.

The numbers of humpback whales and dolphins, normally more abundant off the Southern California coast, have been increasing in Monterey Bay, and now inch-long sea slugs are suddenly concentrating here in spectacular masses, biologists in San Francisco and Santa Cruz have found.

This isn’t El Niño weather, but ocean temperatures along the Northern California coast are higher by several degrees than they have been in decades, and as the warming continues, the immigrant sea slugs are finding it comfortable to thrive farther north.

One species of the southern nudibranchs, popularly known to biology students and home aquarium collectors as the Hopkins’ Rose, is scientifically called Okenia rosacea. Their bright pink bodies are normally common in tide pool areas near Los Angeles and San Diego but uncommon north of San Luis Obispo.

However, they have recently been observed blanketing tide pools as far north as Humboldt County.

Not seen in years

University of California scientists at Santa Cruz, Santa Barbara and Davis’ Bodega Marine Laboratory are reporting large populations of the Hopkins’ Rose species, and they say other types of nudibranchs common in southern waters are also showing up in the Bay Area and as far north as Humboldt County.

“We haven’t seen anything like it in years,” said John Pearse, an emeritus professor of evolutionary biology at UC Santa Cruz.

Gosliner, who has been studying the soft-bodied marine mollusks since he was a Marin County high school student, said the warming water off the California coast is causing the nudibranchs’ move northward, and believes it’s all part of the changing climate observed for decades by scientists around the world.

Nate Mantua, a climate scientist at the NOAA Southwest Fisheries Research Center in Santa Cruz, said sea surface temperatures this winter are averaging about 57 and 58 degrees from the Farallones to Santa Cruz and Monterey Bay, compared with a normal winter range of 52 to 53 degrees.

“It’s the same weather pattern that’s kept the rain from coming and ... causing the drought we’re seeing on land,” he said.

Unusual winter

Normally, Mantua explained, the north winds in the winter strengthen the California current and the upwelling water brings cold water to the surface from deep below. But this winter, he said, those winds have weakened, the upwelling has slowed and the water has warmed.

“We haven’t seen this much warming along the coast since the last El Niño,” Gosliner said. That event marked by warming waters along the coast five years ago was only a moderate one, he noted.

Jeffrey Goddard, a project scientist at UC Santa Barbara’s Marine Science Institute, recalled that he had observed a similar bloom of southern nudibranchs in the winter of 1977 when the El Niño phenomenon was also weak.

But that coincided with a climate shift in the eastern Pacific known as the Pacific Decadal Oscillation, he said, when coastal water temperatures were elevated for two decades. It triggered northward range shifts among species including sea snails and other gastropods, fish, dolphins and barnacles.

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